1. Field of the Invention
The present invention relates to a variable capacitance circuit having an on/off switch for a variable capacitance function, suitably used for communication apparatuses, such as a mobile telephone, a PHS telephone, and a PDA device having a communication function, and to a voltage controlled oscillator using the variable capacitance circuit.
2. Description of the Related Art
Traditionally, in a mobile telephone, a desired frequency signal to be used when transmitting and receiving a data is generated by a PLL synthesizer. The PLL synthesizer includes a reference oscillator which generates a reference frequency signal, and a programmable counter which generates a comparison frequency signal by dividing an output frequency signal outputted from the PLL synthesizer i.e. a frequency signal outputted from a voltage controlled oscillator (VCO: Voltage Controlled Oscillator) by a predetermined frequency dividing ratio.
Further, the PLL synthesizer includes a phase comparator for comparing a phase of the reference frequency signal from the reference oscillator with a phase of the comparison frequency signal from the programmable counter, and outputting a control voltage according to this phase difference, and a voltage controlled oscillator for generating and outputting a frequency signal according to the control voltage from this phase comparator i.e., the frequency signal outputted from the PLL synthesizer.
Although a crystal oscillator with high stability is used as the reference oscillator, it is difficult to oscillate a high frequency signal only with this crystal oscillator. For this reason, this PLL synthesizer generates the comparison frequency signal of relatively low frequency by means of the programmable counter, carries out phase comparison of the comparison frequency signal with the reference frequency signal from the reference oscillator, and obtains a comparison output. The stable high frequency signal is generated by driving the VCO with this comparison output. Further, this PLL synthesizer outputs an output signal of a desired frequency by changing the frequency dividing ratio of the programmable counter.
FIG. 8 shows a capacitance element changeover circuit which is used for the VCO in such a PLL synthesizer and switches between a variable, capacitance element (varactor diode) CV1 for high bandwidth oscillation and a variable capacitance element CV2 for low bandwidth oscillation according to an oscillation frequency band.
If a band changeover control voltage for the high bandwidth oscillation is supplied from a terminal 201 at the time of the high bandwidth oscillation, this band changeover control voltage is supplied to an analog switch 203 through an inverter 202, and the analog switch 203 is turned off.
When the analog switch 203 is turned off, the control voltage for continuously varying frequency, which is supplied through a terminal 204, is supplied only to the variable capacitance element CV1 for the high bandwidth oscillation. Based on a capacitance value of the variable capacitance element CV1, which is variably controlled by this control voltage for continuously varying the frequency, the high frequency signal is oscillated from the VCO.
In addition, at the time of this high bandwidth oscillation, a constant voltage supply switch 205 is turned on by the band change over control voltage for the high bandwidth oscillation, and a constant voltage supplied to a power supply terminal 206 is supplied to the variable capacitance element CV2 for the low bandwidth oscillation instead of the control voltage for continuously varying frequency. With this constant voltage, the capacitance value of the variable capacitance element CV2 for the low bandwidth oscillation is held at a fixed capacitance value, and then the capacitance value of the variable capacitance element CV1 for the high bandwidth oscillation is variably controlled.
Next, at the time of the low bandwidth oscillation, when the band changeover control voltage for the low bandwidth oscillation is supplied from the terminal 201, this band changeover control voltage is supplied through the inverter 202 to the analog switch 203, and the analog switch 203 is turned on.
When the analog switch 203 is turned on, the control voltage for continuously varying frequency, which is supplied through the terminal 204, is supplied to the variable capacitance element CV1 for the high bandwidth oscillation and the variable capacitance element CV2 for the low bandwidth oscillation. Based on the capacitance value of the variable capacitance element CV1 and the capacitance value of the variable capacitance element CV2, which are variably controlled by the control voltage for continuously varying frequency, a low frequency signal is oscillated from the voltage controlled oscillator.
In addition, at the time of this low bandwidth oscillation, the constant voltage supply switch 205 is turned off by the band changeover control voltage for the high bandwidth oscillation, the control voltage for continuously varying frequency is supplied to the variable capacitance element CV2 for the low bandwidth oscillation instead of the constant voltage, and the capacitance value of the variable capacitance element CV2 for the low bandwidth oscillation is variably controlled by the control voltage for continuously varying frequency, as described above.
FIG. 9 shows an example of the voltage controlled oscillator. This voltage controlled oscillator generates a negative resistance component by means of CMOS transistors M1 to M4, and oscillates at a resonance frequency of a parallel resonance circuit constituted by an inductance element L1, an inductance element L2, a variable capacitance element CV (varactor for frequency control), and a band changeover switch Csw (band switch SW). The band changeover switch Csw is controlled according to the band changeover control voltage supplied to a terminal 208.
As for the capacitance value (fosc) of the variable capacitance element CV, its resonance frequency, i.e., the oscillation frequency of the voltage controlled oscillator changes within a range of the following expression, according to a direct current voltage applied to a terminal 207.
In addition, in the following expression, “CVmax” indicates the maximum capacitance value of the variable capacitance element CV, “CVmin” indicates the minimum capacitance value of the variable capacitance element CV, and “Csw” indicates the capacitance value of the band changeover switch.fosc=1/(2π×√L(CVmax+Csw))˜1/(2π×√L(CVmin+Csw))
The band changeover switch Csw is a capacitance switch of a CMOS, as shown in FIG. 10. When obtaining a low bandwidth oscillation frequency, a band switch signal of a low-level voltage value (grounding potential) is supplied to the terminal 208, and when obtaining a high bandwidth oscillation frequency, a band switch signal of a high level voltage value (Vcc) is supplied to the terminal 208, so that the oscillation frequency of the voltage controlled oscillator is changed according to the high bandwidth and the low bandwidth.
In other words, when the band switch signal of the low-level voltage value is supplied, as for the band changeover switch Csw, its capacitance value becomes the maximum value, and an oscillation frequency (foscl) within the frequency range as shown in the following expression is obtained from the voltage controlled oscillator.
In addition, in the following expression, “CVmax” indicates the maximum capacitance value of the variable capacitance element CV, “CVmin” indicates the minimum capacitance value of the variable capacitance element CV, and “Cswh” indicates the capacitance value when the band switch signal of the low-level voltage value is supplied to the band changeover switch, i.e., the maximum capacitance value.foscl=1/(2π×√L(CVmax+Cswh))˜1/(2π×√L(CVmin+Cswh))
Similarly, when the band switch signal of the high-level voltage value is supplied, the capacitance value of the band changeover switch Csw becomes the minimum value, and an oscillation frequency (fosch) within the frequency range as expressed by the following expression is obtained from the voltage controlled oscillator.
In addition, in the following expression, “CVmax” indicates the maximum capacitance value of the variable capacitance element CV, “CVmin” indicates the minimum capacitance value of the variable capacitance element CV, and “Cswl” indicates the capacitance value when the band switch signal of the high-level voltage value is supplied to the band changeover switch, i.e., the minimum capacitance value.fosch=1/(2π×√L(CVmax+Cswh))˜1/(2π×√L(CVmin+Cswh))[Patent Document 1];
Japanese Patent Application Publication No. 2002-43842 (pp 3–4: FIG. 1)